1. Field of the Invention
The invention relates to a photoacoustic detector that is constructed as a cylinder and in which the second azimuthal resonance oscillation is used to enhance the acoustic signal.
2. Discussion of Background Information
Photoacoustic measuring methods, i.e., measuring methods in which the gas that is to be investigated is irradiated with a light source and heated by absorption, are highly suitable for precise measurements of the concentration of absorbent substances, especially absorbent substances in gases. The gas expands when it is heated. If heating, and hence expansion, are periodic, a sound wave is formed and this sound wave can be measured by a sound pressure sensor.
Photoacoustic spectroscopy has several advantages over classic absorption spectroscopy in which the light passing through the sample is measured, and the absorption is inferred from the difference between the incoming light and the light passing through the sample. The photoacoustic signal is linear within a concentration range of approximately 5-6 orders of magnitude. The sensitivity of the detector is independent of the wavelength of the excitation light. A photoacoustic detector with comparable sensitivity is smaller and cheaper than a classic optical absorption spectroscopy detector.
One problem with photoacoustic measurement is that the photoacoustic signal is proportional to the output of the incoming light. The output of the diode lasers, or quantum cascade lasers, normally used is insufficient for the sensitive measurement of some substances. In optical absorption spectroscopy, the approach of lengthening the light path is used to enhance sensitivity. So-called multipass detectors are used. In these detectors, the light is reflected multiple times through the measuring area. Mirrors that are correspondingly arranged are used for this purpose. After several reflections, the light beam is guided out of the measuring cell and directed towards a detector. To prevent interference in the measuring cell, the light beam is guided as a straight line folded together between the two windows from the inlet window to the exit window.
To increase the measuring sensitivity of photoacoustic reflectors, a multipass arrangement can also be chosen in which the excitation light is reflected multiple times across the measuring range. Approaches of this type are described in A. Miklos, J. Ng, P. Hess, A. H. Kung, “Application of a wavelength-amplitude double-modulation method in photoacoustic detection using a pulsed optical parametric oscillator,” Journal de Physique IV, 125, 579-582, (2005), and A. Miklos, S-C. Pei and A. H. Kung, “Multipass acoustically open photoacoustic detector for trace gas measurements,” Applied Optics 45, 2529-2534, (2006). One problem with such approaches is that the alignment of the multipass arrangement has to be relatively exact. Up to now, this has delayed the development of practical photoacoustic multipass detectors.
A photoacoustic measuring arrangement with an acoustic measuring cell is known from U.S. Pat. No. 3,938,365. Here, the excitation is generated by largely monochromatic radiation that causes pressure fluctuations which lead to standing waves in the measuring cell. The intensity of the excitation light is modulated thereby, with the modulation frequency corresponding to one of the natural acoustic oscillations. This document also discloses that longitudinal, radial and azimuthal modes can be excited in the cylindrical measuring cell.